Stereo cameras capable of measuring the distance to an object are used. For example, a technology has been put into practical use to control a car by measuring the distance to an object in front of the car by using a stereo camera (hereafter, referred to as a “car-mounted stereo camera”) that is mounted on a car. For example, the distance that is measured by a car-mounted stereo camera is used for giving an alarm to a driver or controlling a brake, steering, and/or the like, for the purpose of prevention of car collision, a control on a distance between cars, and/or the like.
Generally, car-mounted stereo cameras are often installed inside the front windshield of cars. This is because, if a car-mounted stereo camera is installed outside the car, it needs to have a higher durability in terms of waterproof, dust prevention, and the like. If a stereo camera is installed inside the car, it captures an image of the landscape outside the car through the front windshield. Generally, the front windshield has a complicated curved shape, and it has a distorted shape compared to optical components such as a lens inside the camera. Therefore, the front windshield causes the captured image that is captured through the front windshield to be distorted. Furthermore, depending on the installation position and the installation direction when installing a stereo camera in the car, the characteristics of a distortion of captured images are changed. To calibrate such a distortion that is included in the captured image, it is necessary to calculate a calibration parameter for calibrating (correcting) a distortion of the captured image by installing a stereo camera in a predetermined position of the vehicle and then using the captured image that is captured through the front windshield in that state.
A generally known method for calculating a calibration parameter for calibrating a distortion of a captured image is a method that uses a chart in which a specific mark, or the like, for measuring a distance is described. With this method, a calibration parameter for calibrating a distortion of the captured image is calculated on the basis of the difference between the position of a mark (object) that is on the captured image and that is calculated in theory on the basis of the relative position between the mark and the stereo camera and the position of the mark on the captured image that is obtained when an image of the mark is actually captured by the stereo camera. That is, a calibration parameter is calculated, which determines a conversion so as to eliminate the difference.
Patent Literature 1 discloses a device that converts each of a pair of image data sets output from a pair of cameras that is included in a stereo camera by using a calibration parameter that is based on the difference in the coordinates between one set of the image data and the other set of the image data so as to adjust an optical distortion and a positional deviation of the stereo camera by image processing.
However, if there is an error in the relative position between the stereo camera and the chart, an error occurs in the coordinates of the object that is in the captured image and that is calculated in theory; therefore, an error also occurs in a calibration parameter for calibrating a distortion of the captured image. Particularly, there is a problem in that an error easily occurs in the relative position between the chart and the stereo camera that is installed in an object, such as a vehicle.
In consideration of the foregoing, there is a need to provide a calibration method and a measurement tool that make it possible to calculate a high-accuracy calibration parameter for calibrating a stereo camera.